Tubing conveyed perforating systems have been widely used during the past decade for economically perforating a well. As disclosed in U.S. Pat. Nos. 4,299,287 and 4,480,690, a perforating gun may be conveyed to its desired depth in the wellbore from a tubular string, the perforating gun fired, and the produced fluid recovered at the surface via the same tubing string. Those skilled in hydrocarbon recovery recognize that tubing conveyed perforating has particular advantages for many wells compared to perforating systems which suspend the perforating gun in the wellbore from a wireline. Perforating guns may be percussion fired by dropping a weight through the tubular string to activate the gun, or may be hydraulically fired by increasing downhole pressure to rupture a fluid barrier disk and thereby fire the gun.
Many oil recovery operations benefit from a gravel packing technique whereby gravel is placed in the borehole across the perforated area of the well casing and is forced into the formation a short distance through the perforated casing. Gravel packing filters out sand produced with the fluids, thereby minimizing sand production. U.S. Pat. Nos. 4,450,051 and 4,541,486 disclose a combined perforating and gravel packing operation which may be performed in one "trip" of the tubular and gravel packing operation which may be performed in one "trip" of the tubular string, thereby reducing the time and cost of the completion process. Toward the end of the gravel packing operation, a one-way flapper valve may be used to prevent pressurized fluid in the casing annulus from entering the bypass system around a crossover port and thereby enter the formation, as disclosed in the '051 patent. U.S. Pat. No. 4,846,281 discloses a gravel packing tool with dual flapper valves which may be used to permit a subsequent well logging operation to be performed while the tool remains within the wellbore. The dual flapper valve arrangement also conserves completion fluid, and allows the producing formation to be protected from the pressure of the column of heavy completion fluid during retrieval of the work string and installation of the production string. U.S. Pat. No. 4,813,481 discloses further particulars with respect to a flapper valve and a sealing seat used in downhole tools.
While the one trip perforating and gravel packing technique as disclosed in the above patents has reduced the cost of completion in various applications, this technique has some drawbacks which have limited its acceptance in the industry. In some cases, the shock generated by firing the perforating guns may actuate or damage other tools in the completion string. Various types of shock absorbers have been placed in the tool string above the perforating guns to minimize this problem. These shock absorbers systems are not always effective and inherently increase the cost of the completion operation. In other applications using tubing conveyed perforating gun, tools may need to be isolated from the gun shock.
Those skilled in gravel packing and perforating operations also appreciate the substantial damage which may be caused by subjecting the formation to well completion fluids during the completion operation. After gravel packing, the completion fluid may be circulated to the surface through the tubing to both avoid squeezing the gravel pack and to displace the service fluid used during well treatment. The pressure required for circulating the completion fluid may thus adversely affect the subsequent production operation. Moreover, hundreds of barrels of completion fluid may be lost into a well, and the cost of the base completion fluid and the fluid additives are a significant cost to the overall completion operation. While techniques as disclosed in U.S. Pat. No. 4,846,281 have been devised to both minimize formation damage and prevent loss of completion fluids in the annulus of the wellbore after gravel packing, large quantities of completion fluids are still lost in many tubing conveyed perforating operations, and this lost fluid adversely affects the efficiency of the oil recovery operation.
The disadvantages of the prior art are overcome by the present invention, and improved techniques are hereinafter disclosed for a tubing conveyed perforating operation which both minimizes the possible damage to the formation and minimizes the loss of expensive completion fluids.